An integrated circuit chip assembly generally comprises an integrated circuit chip attached to a substrate, typically a chip carrier or a circuit board. The most commonly used integrated circuit chip is composed primarily of silicon having a coefficient of thermal expansion of about 2 to 4 ppm/° C. The chip carrier or circuit board is typically composed of either a ceramic material having a coefficient of thermal expansion of about 6 ppm/° C., or an organic material, possibly reinforced with organic or inorganic particles or fibers, having a coefficient of thermal expansion in the range of about 6 to 50 ppm/° C. One technique well known in the art for interconnecting integrated circuit chips and substrates is flip chip bonding. In flip chip bonding, a pattern of solder balls is formed on the active surface of the integrated circuit chip, allowing complete or partial population of the active surface with interconnection sites. The solder balls which typically have a diameter of about 0.002 to 0.006 inches, are deposited on solder wettable terminals on the active surface of the integrated circuit chip forming a pattern. A matching footprint of solder wettable terminals is provided on the substrate. The integrated circuit chip is placed in alignment with the substrate and the chip to substrate connections are formed by reflowing the solder balls. Flip chip bonding can be used to attach integrated circuit chips to chip carriers or directly to printed circuit boards. The terminals located on the side of the substrate facing the integrated circuit chip are in turn interconnected to connecting balls or pins on the opposite side of the substrate in a well known manner in order to facilitate the external connection of the assembly to contacts or terminals on, for example, a circuit board.
A feature of established practices in the integrated circuit industry, provides that the substrate with the attached integrated circuit chip are formed into a package by encapsulating the assembly into a unitary package. This provides physical and environmental protection for the delicate integrated circuit chip including isolating the integrated circuit chip and the interconnections from moisture. It also provides firm bonding between the integrated circuit chip and the substrate to thereby prevent relative movement between them and the potential disruption of the interconnections. During operation of an integrated circuit chip assembly, cyclic temperature excursions cause the substrate and the integrated circuit chip to expand and contract. Since the substrate and the integrated circuit chip have different coefficients of thermal expansion, they expand and contract at different rates, possibly causing the solder ball connections to weaken or even crack as a result of fatigue. To remedy this situation, it is common industry practice to reinforce the solder ball connections with a thermally curable polymer material known in the art as an underfill encapsulant.
Underfill encapsulants have been widely used to improve the fatigue life of integrated circuit chip assemblies consisting of an integrated circuit chip of the flip chip variety attached to a substrate made of alumina ceramic material having a coefficient of thermal expansion of about 6 ppm/° C. More recently, integrated circuit assemblies having an integrated circuit chip of the flip chip type attached to a substrate made of a reinforced organic material with a composite coefficient of thermal expansion of about 20 ppm/° C. have been manufactured.
During the packaging of the integrated circuit attached to the substrate, the underfill encapsulation process is typically accomplished by dispensing the liquid encapsulant at one or more points along the periphery of the integrated circuit chip. The encapsulant is drawn into the gap between the integrated circuit chip and the substrate by capillary forces, substantially filling the gap and forming a fillet around the perimeter of the integrated circuit chip. An example of such an underfilling method is described in U.S. Pat. No. 5,817,545, entitled “Pressurized Underfill Encapsulation Of Integrated Circuits”, which issued Oct. 6, 1998.
The diameter of the filler particles in the encapsulant are sized to be smaller than the height of the gap so as not to restrict flow. Typical encapsulant formulations have a viscosity of about 10 Pa-s at the dispense temperature. After the encapsulant has flowed into the gap, it is cured in an oven at an elevated temperature.
Cured encapsulants typically have coefficients of thermal expansion in the range of about 20 to 40 ppm/° C., and a Young's Modulus of about 1 to 3 GPa, depending on the filler content and the polymer chemistry. It may be desirable in some cases to further alter the cured properties of the encapsulant, however, the requirement that the encapsulant have low viscosity in the uncured state severely restricts the formulation options. For example, the addition of more ceramic filler would lower the resulting coefficient of thermal expansion, but increase the uncured viscosity.
Known in the art are methods for encapsulation of a flip chip package wherein a package body is formed around the perimeter of the flip chip in a two step process. First the integrated circuit chip is underfilled as previously described for the packaging, and then a package body is formed around the perimeter of the integrated circuit chip using a molding process. In yet another known method, additional reinforcement is achieved by encapsulating both faces of the flip chip and its perimeter in a single step. In this technique, the gap between the integrated circuit chip and the substrate has been substantially eliminated by forming a hole in the substrate that comprises a significant portion of the active area of the integrated circuit chip. This approach essentially eliminates the small gap typical of a conventional integrated circuit chip to substrate interconnection, but has the drawback of limiting the active area of the integrated circuit chip that can be used for forming interconnections because only the perimeter of the integrated circuit chip can be used. Examples of descriptions of injection encapsulation making use of an opening in the substrate below the integrated circuit chip in order to encapsulate the interconnections are described in U.S. Pat. No. 6,081,997, entitled “System and Method For Packaging An Integrated Circuit Using Encapsulant Injection”, which issued Jul. 4, 2000 and U.S. Pat. No. 5,981,312, entitled “Method For Injection Molded Flip Chip Encapsulation”, which issued Nov. 9, 1999.
Another example of attempts to improve the encapsulation of integrated circuit packages is described in European patent application EP 1075022, entitled “Offset Edges Mold For Plastic Packaging Of Integrated Semiconductor Devices”, which was published Feb. 7, 2001. In this application the description includes causing and directing the flow of plastic resin to the more restricted areas into the depressed central areas of the mold below where the device is located in a cavity as well as the upper part of the cavity above the device.
Notwithstanding the use of known underfill encapsulation techniques, fatigue life of an integrated circuit chip assembly may be shorter when the solder interconnections are made to organic substrates as opposed to ceramic substrates, owing to the greater mismatch in thermal expansion. Together with the limitations imposed on formulation options by the low viscosity requirement, improvements in encapsulation techniques and the mechanical reinforcement of integrated circuit chip interconnections are still required.